This invention relates to ladle refining of steel. It has particular, but not exclusive, application to the ladle refining of steel to be directly cast into thin steel strip in a continuous strip caster.
It is known to cast metal strip by continuous casting in a twin roll caster. In such a process, molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product which is delivered downwardly from the nip between the rolls. The molten metal may be introduced into the nip between the rolls via a tundish and a metal delivery nozzle located beneath the tundish so as to receive a flow of metal from the tundish and to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls.
Twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, for example aluminum. However, there have been problems in applying the technique to the casting of ferrous metals. One particular problem has been the propensity for ferrous metals to produce solid inclusions which clog the very small metal flow passages required in a twin roll caster.
The use of silicon-manganese in ladle deoxidation of steel was practiced in ingot production in the early days of Bessemer steelmaking and as such the equilibrium relations between the reaction product molten manganese silicates and the residual manganese, silicon and oxygen in solution in steel are well known. However in the development of technology for the production of steel strip by slab casting and subsequent cold rolling, silicon/manganese deoxidation has generally been avoided and it has been considered necessary to employ aluminum killed steels. In the production of steel strip by slab casting and subsequent hot rolling followed often by cold rolling, silicon/manganese killed steels produce an unacceptably high incidence of stringers and other defects resulting from a concentration of inclusions in a central layer of the strip product.
In the continuous casting of steel strip in a twin roll caster, it is desirable to generate a finely controlled flow of steel at constant velocity along the length of the casting rolls to achieve sufficiently rapid and even cooling of steel over the casting surfaces of the rolls. This requires that the molten steel be constrained to flow through very small flow passages in refractory materials in the metal delivery system under conditions in which there is a tendency for solid inclusions to separate out and clog those small flow passages.
After an extensive program of strip casting various grades of steel in a continuous strip roll caster we have determined that conventional aluminum killed carbon steels or partially killed steel with an aluminum residual content of 0.01% or greater generally cannot be cast satisfactorily because solid inclusions agglomerate and clog the fine flow passages in the metal delivery system to form defects and discontinuities in the resulting strip product. This problem can be addressed by calcium treatment of the steel to reduce the solid inclusions but this is expensive and needs fine control, adding to the complexity of the process and equipment. On the other hand, it has been found that it is possible to cast strip product without stringers and other defects normally associated with silicon/manganese killed steels because the rapid solidification achieved in a twin roll caster avoids the generation of large inclusions and the twin roll casting process results in the inclusions being evenly distributed throughout the strip rather than being concentrated in a central layer. Moreover, it is possible to adjust the silicon and manganese contents so as to produce liquid deoxidation products at the casting temperature to minimize agglomeration and clogging problems.
In conventional silicon/manganese deoxidation processes, it has not been possible to lower free oxygen levels in the molten steel to the same extent as is achievable with aluminum deoxidation and this in turn has inhibited desulphurization. For continuous strip casting, it is desirable to have a sulphur content of the order of 0.009% or lower. In conventional silicon/manganese deoxidation processes in the ladle, the desulphurization reaction is very slow and it becomes impractical to achieve desulphurization to such low levels particularly in the case where the steel is produced by the electric arc furnace (EAF) route using commercial quality scrap. Such scrap may typically have a sulphur content in the range 0.025% to 0.045% by weight. The present invention enables more effective deoxidation and desulphurization in a silicon/manganese killed steel and refining of high sulphur steel in a silicon/manganese killed regime to produce low sulphur steel suitable for continuous thin strip casting.
According to an illustrative embodiment of the invention there is provided a method of refining steel in a ladle, including heating a steel charge and slag forming material in a ladle to form molten steel covered by a slag containing silicon, manganese and calcium oxides, and stirring the molten steel by injecting an inert gas into it to cause silicon/manganese deoxidation and desulphurization of the steel to produce a silicon/manganese killed molten steel having a sulphur content of less than 0.01% by weight.
The molten steel may have a free oxygen content of no more than 20 ppm during the desulphurization.
The free oxygen content during desulphurization may for example be of the order of 12 ppm or less.
The inert gas may for example be argon.
The inert gas may be injected into a bottom part of the molten steel in the ladle at a rate of between 0.35 scf/min to 1.5 scf/min per ton of steel in the ladle so as to produce a strong stirring action promoting effective contact between the molten steel and the slag.
The inert gas may be injected into the molten steel through an injector in the floor of the ladle and/or through at least one injection lance extended downwardly into the bottom part of the metal in ladle.
The molten steel may have a carbon content in the range 0.001% to 0.1% by weight, a manganese content in the range 0.1% to 2.0% by weight and a silicon content in the range 0.1% to 10% by weight.
The steel may have an aluminum content of the order of 0.01% or less by weight. The aluminum content may for example be as little as 0.008% or less by weight.
The molten steel produced by the method of the present invention may be cast in a continuous thin strip caster into thin steel strip of less than 5 mm thickness.
Heating of the ladle may be carried out in a ladle metallurgical furnace (LMF). The LMF may have several functions, including:
1. Heat the liquid steel in the ladle to the required exit temperature that is suitable for subsequent processing such as a continuous casting operation.
2. Adjust the steel composition to the specific requirements of the following process.
3. Achieve reduction of the sulphur content of the steel to the aim final sulphur content.
4. Achieve thermal and chemical homogeneity in the liquid steel bath.
5. The agglomeration and floatation of oxide inclusions and their subsequent capture and retention in the refining slag.
In a conventional ladle metallurgical furnace (LMF), the heating may be achieved by electric arc heaters. The liquid steel must be covered with a refining slag weight and a gentle forced circulation is required for temperature homogeneity. This is achieved by electromagnetic stirring or gentle argon bubbling. The weight and thickness of the slag is sufficient to enclose the electric arcs, and whose composition and physical characteristics (i.e., fluidity) are such that the slag captures and retains sulphur and solid and liquid oxide inclusions which result from deoxidation reactions and/or reaction with atmospheric oxygen.
The molten steel may be stirred by injection of an inert gas such as for example argon or nitrogen to facilitate slag-metal mixing in the ladle and desulphurization of the steel. Typically, the inert gas may be injected through a permeable refractory purging plug located in the bottom of the ladle or through a lance. We have now determined that if an unusually strong or violent stirring action is achieved, for example by injection of argon through a lance that is dipped into the steel, in conjunction with a slag regime rich in CaO it is possible to achieve remarkable non-equilibrium outcomes such as very low steel free oxygen levels with silicon deoxidation. In particular, it is possible readily to achieve free oxygen levels of the order of 10 ppm as opposed to an expected result of 50 ppm. This low free oxygen content enables more effective desulphurization and it becomes possible to achieve very low sulphur levels in a silicon/manganese killed steel.
Specifically, we have determined that by injecting argon through a lance at flow rates of 0.35 scf/min to 1.5 scf/min per ton of molten steel with a liquid slag high in CaO it is possible to achieve free oxygen in a silicon/manganese regime at 1600xc2x0 C. of less than 12 ppm and as low as 8 ppm and to rapidly achieve desulphurization to sulphur levels of below 0.009%. It is believed that the violent stirring of the molten metal promotes mixing between the liquid slag and the steel and promotes removal of SiO2, which is the product of the reaction of silicon with free oxygen in the steel, thereby promoting continuation of the silicon deoxidation reaction to produce low free oxygen levels more conventionally expected with aluminum deoxidation.
At the conclusion of the desulphurization step, the slag may be thickened to prevent reversion of sulphur back into the steel, and then oxygen injected into the steel to increase the free oxygen content to between about 40 ppm and about 70 ppm and generally about 50 ppm so as to produce a steel that is readily castable in a twin roll caster.